The purpose of this invention is to describe propellant grains which demonstrate significantly better ignition characteristics than state-of-the-art propellant grains.
In the design of devices incorporating a propellant, and particularly those devices that use the burning propellant gases to accomplish mechanical work, such as gun ammunition, a controlled amount of gas and heat must be released within a specified time period by the burning propellant.
With existing propellants, for example those based on nitrocellulose, nitrocellulose/nitroglycerine, and nitrocellulose/nitroglycerine/nitroguanidine, and bettern known to those practices in the art as "single base", "double base", and "triple base" propellants, respectively, the burning rate of the propellant is fixed within rather narrow limits by the formulation; a major change in propellant formulation is required to significantly alter such characteristics as burning rate. As a result, to control the release of heat and gas in a device such as a rocket motor chamber, gun chamber, or other gas generating device, the propellant is configured into a sometimes complicated three dimensional geometry, known as a "grain", so that the amount of surface burning at a given time is controlled. The inherent burning rate of the propellant and the grain size and geometry determine the burning rate or "quickness" of the propellant mass. In many instances, a substance is used in the outer layer of the grain to retard the initial burning rate, and is known as a "deterrent layer." Additionally, most propellant grains designed for use in guns incorporate an outer layer of graphite glazed onto the propellant surface; needed to protect the grain and reduce the handling hazards due to friction and static electricity.
The function sequence of the propellant burning can be discussed in terms of two events: (1) ignition of the exterior surface of the grain, and (2) regressive burning of the propellant grain. The first event, ignition, depends on the propagation of a flame front, generated by a priming source and burning of the propellant ignited first, through the propellant bed or along a monolithic grain. In a loosely packed bed of individual propellant grains, this gaseous flame front propagates easily through the interstitial voids in the bed. However, due to the deterrents and coatings on the grain, and in some cases, the nature of the propellant itself, the actual ignition of the propellant matrix by the hot gas is retarded, particularly at low temperature. It is additionally believed by those practiced in the art that a certain amount of hot particles, i.e., condensed chemical species in the flame, contribute substantially to the ignition event, in addition to the hot gases; the propagation of such hot particles through a propellant bed is difficult compared to the gas propagation.
This invention describes propellant grains otherwise similar to state-of-the-art propellant grains, but fabricated to incorporate an ignition compound or burning rate enhancing layer as a coating or as a chemically bound surface layer on each propellant grain, and thus dispersed uniformly along each monolithic grain that comprises a propellant bed. The burning rate enhancer, specifically consisting of selected compounds based on certain decahydrodecaborate (-2) salts, greatly facilitates propagation of the initiating flame front along the propellant surface, and/or through the propellant bed. The propellants described by this invention demonstrate significantly better ignition and burning characteristics than similar state-of-the-art propellant grains at ambient temperature; the improvement is even more marked at low temperature.